Strategies for controling attribute assignment

[Dale Strickland-Clark]

"Steve Holden" <sholden at holdenweb.com> wrote:

>"Dale Strickland-Clark" <dale at riverhall.NOSPAMco.uk> wrote in message
>news:9lajrtcqsojv72nr88c7j6kmdapjo44vc0 at 4ax.com...
>> A 'properly' encapsulated business class tends to involve methods and
>> attributes. However, managing attribute assignment gets very messy
>> with __setattr__ as you need to devise a scheme to distinguish class
>> attributes and working instance variables.
>>
>> Often, externally visible class attributes need to be validated and
>> instance variables don't. Or they may get stored elsewhere.
>>
>> What strategies have people come up with to manage this distinction
>> without bogging the code down in excessive checks.
>>
>> I would really like a way to distinguish between these two without
>> having to check dictionaries.
>>
>Dale:
>
>I'm not quite sure what problem you are trying to solve here. Are you
>suggesting that your code will change some class attributes and some
>instance attributes to parameterize the operation of all instances for a
>particular context. Maybe I need another cup of coffee ...
>
>Well, the coffee was a good idea. Your problem is that when you *use*
>__setattr__ you need to know whether you are setting a class or an instance
>variable, right? And you want to set class attributes from inside methods?
>
>Perhaps you'd explain what you mean by "proper" encapsulation is a problem
>in your particular context. I'm presuming that you are working in a COM
>environment, but of course I've been wrong before...
>
>regards
> Steve
>
>PS: See recent thread on not hiding your email address from spam!

I thought this might be difficult to convey. This is typical of the
approach I'm currently using:

	def __setattr__(self, field, value):
		if self._dbTable.has_key(field):
			self._dbTable[field].validate(value)
			self._fields[field] = value
			self._saved = 0	
		else:
			self.__dict__[field] = value
		
I have to distinguish between assignments to 'public' attributes
(which will go in my database) and instance variables which are used
throughout the class. I need to validate assignments to the database
but just let everything else through.

But it means that EVERY internal assignment to instance attributes
needs to go through this code.

I guess what I'm after is a distinction between private and public
attrbutes.

Does that make it any clearer?
--
Dale Strickland-Clark
Riverhall Systems Ltd